Tunneling spectroscopy of layered superconductors: intercalatedLi 0.48 (C 4 H 8 O) x HfNCl and De-intercalated HfNCl 0.7 (original) (raw)
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Nova Science Monograph “Superconductivity: Theory, Materials and Applications”, Ed. Vladimir R. Romanovskii, Chapter 1, pp. 1-110, 2012
The discovery of novel high-T c superconductivity in MgB 2 (T c = 39.5 K) and Li0.48(THF)yHfNCl (Tc = 25.5 K) initiated substantial progress in the field of superconductivity physics and its applications despite the fact that competing high-Tc cuprates remain the world leaders in almost all practically important superconducting parameters. This article describes electron tunneling and point-contact experimental studies of the indicated two materials and related substances, being crucial to elucidate the character of the quasiparticle energy spectrum both in superconducting and normal state. The account is based mostly on our own experiments, although works carried out in other laboratories are taken into account as well.
Gap features of layered iron-selenium-tellurium compound below and above the superconducting transition temperature by break-junction spectroscopy combined with STS Abstract. We studied correlations between the superconducting gap features of Te-substituted FeSe observed by scanning tunnelling spectroscopy (STS) and break-junction tunnelling spectroscopy (BJTS). At bias voltages outside the superconducting gap-energy range, the broad gap structure exists, which becomes the normal-state gap above the critical temperature, T c. Such behaviour is consistent with the model of the partially gapped density-wave superconductor involving both superconducting gaps and pseudogaps, which has been applied by us earlier to high-T c cuprates. The similarity suggests that the parent electronic spectrum features should have much in common for these classes of materials. 1. Introduction From the early stages of our break-junction tunnel spectroscopy (BJTS) measurements on the superconducting FeSe x Te 1-x single crystals, the normal-state gaps were manifested in addition to the Bardeen-Cooper-Schrieffer (BCS)-type superconducting gap features [1,2]. The observed unusual electronic properties of this superconductor including the gap distributions extending to several tenth of meV are in fact intriguing in view of the possible manifestation of the local anomalously high (about 100 K [3]) superconducting critical temperature (T c). On the other hand, it is reasonable to consider that the normal-state gap is a consequence of the charge-or spin-density-wave formation below the structural/magnetic phase transition, as stems from the phase diagram [4]. In particular, there has been suggested that the lower symmetry C 2 nematic/smectic charge ordering emerges in the iron-based superconductors [5,6]. In such states, the normal-state gap structures can be readily observed by tunnelling spectroscopy, which was proposed theoretically [7]. However, there were not enough direct tunnel measurements of such a normal-state gap. In this paper, we report the observations of both superconducting and the normal-state gaps for the simple iron-based superconductor, FeSe x Te 1-x , using the BJTS techniques as well as the scanning tunnel spectroscopy (STS). We emphasize that the gap of ± 10-20 mV observed by STS in the superconducting state below T c = 15 K is larger than that revealed in the BJTS measurements [1]. The attention should be paid to the fact that these measurements constitute the most direct and precise tool among the gap-probing experimental techniques. This probe dealing with the conduction electrons themselves immediately leads to the well energy-resolved data needing no further assumptions and relatively easy in interpretation [8].
2012
The discovery of novel high-T c superconductivity in MgB 2 (T c = 39.5 K) and Li0.48(THF)yHfNCl (Tc = 25.5 K) initiated substantial progress in the field of superconductivity physics and its applications despite the fact that competing high-Tc cuprates remain the world leaders in almost all practically important superconducting parameters. This article describes electron tunneling and point-contact experimental studies of the indicated two materials and related substances, being crucial to elucidate the character of the quasiparticle energy spectrum both in superconducting and normal state. The account is based mostly on our own experiments, although works carried out in other laboratories are taken into account as well.
Energy gap structure and tunneling characteristics of layered superconductors
Physica C: Superconductivity, 1993
The submitted manuscript has been authored by a contractor of the U. S. Government under contract No. W-31-109-ENG-38. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution. or allow others to do so, for U.S. Government pupa ses.
Atomic site tunneling spectroscopy on high-Tc superconductors
Physica B: Condensed Matter, 1994
Superconducting gap structures of BizSr2CaCu2Oy (BSCCO) and YBa2Cu3Oy (YBCO) have been probed by scanning tunneling microscopy (STM) at cryogenic temperatures. The tunneling conductance curves observed on bulk single crystals of BSCCO and epitaxial thin films of YBCO revealed clear overshooting peaks and flat bottom regions around V= 0 with quite low zero-bias conductances of -1%. Since the electron tunneling process in STM is essentially incoherent, the present observation is favored by the s-wave pairing mechanism. However, the conductance curves were found to be substantially smeared in comparison with the conventional excitation spectra predicted in the BCS (isotropic s-wave) superconductors, suggesting gap anisotropy. 0921-4526/94/$07.00 ~ 1994 Elsevier Science B.V. All rights reserved SSDI: 0921-4526(93)E0192-J
Journal of Physics: Conference Series, 2014
The layered superconductors β-MNCl with the critical temperatures T c = 14 K (M = Zr) -25 K (M = Hf) were investigated by means of scanning-tunneling microscopy/spectroscopy and break-junction tunneling spectroscopy. The STM/STS was used to investigate the surface electronic structures in nanometer length scale, while the BJTS was employed to precisely determine the gap characteristics. Both techniques consistently clarified the unusually large size of the superconducting gap. Wide gap distributions with large-scale maximum gap values were also revealed in α-K y TiNCl with a different crystal structure.
Tunneling and infrared spectroscopy on high Tc superconductors
Journal of Physics and Chemistry of Solids, 1993
We have investigated single crystais of Bi$k2Cat,YxC$Oe (x=0 -0.35) by far infrared transmission spectroscopy and by break junction tmmeling. The junctions were made at low temperatures in vacuum, and SIS tunneling was measured at various temperatures and magnetic fields. 'IEe tmmeling resuits are interpreted in terms of a Josephson current and a single electron tunneling density of states (DOS). Although the DOS is found to he zerc at the Fermi level, we do not see a fully developed gap even at temperatures well below the critical temperature.
Tunneling spectroscopy of the superconducting energy gap inRNi2B2C(R=YandLu)
Physical Review B, 1996
Superconducting energy gap in RNi 2 B 2 C ͑RϭY and Lu͒ has been investigated using a break-junction tunneling technique. The observed differential conductance dI/dV is well described by the BCS density of states with a small lifetime broadening parameter. We have found small gap values for the local low-T c phases in addition to the bulk T c of 15.6-16.6 K. The gap value of 2⌬ϭ4.5 meV with T c ϭ15 K gives the BCS ratio of 2⌬/k B T c ϭ3.5, which is kept constant down to the observed lowest T c of 4.6 K. These results give clear evidence for the BCS mechanism of superconductivity for these compounds.
Tunneling spectroscopy: A probe for high-Tc superconductivity
Microelectronics Journal, 2008
After the pioneering work of Giaever [Phys. Rev. Lett. 5 (1960) 147. ] and Shapiro in the 1960s, tunneling spectroscopy has become an important method for studying different properties of superconductors, such as the superconducting gap and the density of states. At the interface with a superconductor there is a variety of tunneling processes depending on barrier properties and the type of materials in both sides of the junction. Among other phenomena, we have single electron tunneling, cooper pairs (Josephson) tunneling [B.D. Josephson, Phys. Rev. Lett. 1 (1962) 251. [17]] and mixed (Andreev-Saint-James reflections [A.F. Andreev, Zh. Eksp. Teor. Fiz. 46 (1964) 1823, Saint-James D., J. Phys 25 (1964) 899]) tunneling.
Physical Review B, 2017
Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on a multi-layered cuprate superconductor Ba2Ca5Cu6O12(O1−x,Fx)2. STM topography revealed random spot structures with the characteristic length ≤ 0.5 nm. The conductance spectra dI/dV (V) show the coexistence of smaller gaps ∆S and large gaps (pseudogaps) ∆L. The pseudogap-related features in the superconducting state were traced with the spatial resolution of ∼ 0.07 nm. Here, I and V are the tunnel current and bias voltage, respectively. The temperature, T , dependence of ∆S follows the reduced Bardeen-Cooper-Schrieffer (BCS) dependence. The hallmark ratio 2∆S(T = 0)/kBTc equals to 4.9, which is smaller than those of other cuprate superconductors. Here, Tc is the superconducting critical temperature and kB is the Boltzmann constant. The larger gap ∆L survives in the normal state and even increases with T above Tc. The T dependences of the spatial distributions for both relevant gaps (∆ map), as well as for each gap separately (∆S and ∆L) were obtained. From the histogram of ∆ map, the averaged gap values were found to be∆S =∼ 24 meV and∆L =∼ 79 meV. The smaller gap ∆S shows a spatially homogeneous distribution while the larger gap ∆L is quite inhomogeneous, indicating that rather homogeneous superconductivity coexists with the patchy distributed pseudogap. The spatial variation length ξ∆ L of ∆L correlates with the scale of the topography spot structures, being approximately 0.4 nm. This value is considerably smaller than the coherence length of this class of superconductors, suggesting that ∆L is strongly affected by the disorder of the apical O/F.